To jest łóżko, które kupiłem w listopadzie.

This is the bed that I bought in November.

Breakdown of To jest łóżko, które kupiłem w listopadzie.

• ja: I
• być: to be
• to: this
• w: in

Questions & Answers about To jest łóżko, które kupiłem w listopadzie.

What does To jest mean, and why do we use to here instead of ten/ta/to or other demonstratives?

To jest literally means “this is” (or “that is” in some contexts). It’s a fixed phrase used to introduce or point at something. The word to here is a neuter demonstrative pronoun and is always used in To jest … regardless of the gender or number of the noun you’re introducing.

You do not say Ten jest or Ta jest in this construction—only To jest.

Why is there a comma before które? Do I always need a comma before relative clauses in Polish?

Yes. In Polish, any clause introduced by a relative pronoun (takim jak który, która, które, którzy, których) is set off by a comma. So you always write
To jest łóżko, które kupiłem w listopadzie.
The comma marks the start of the subordinate (relative) clause.

What is które and how do I choose the correct form of który (który, która, które, etc.)?

Który means “which” or “that” and agrees in gender, number, and case with its antecedent (the noun it refers to).

• Gender: masculine/feminine/neuter
• Number: singular/plural
• Case: depends on its role in the relative clause

Here, łóżko is neuter singular, and inside the clause it’s the direct object of kupiłem, so it’s in accusative. The neuter singular accusative form of który is które (which happens to look the same as the nominative).